GH-1244: Change modp_dtoa2() to use scientific notation for small values

This fixes problems where printing floating point numbers less than
10^-6 output as "0.0".  Such numbers now use using scientific notation
and preserve the value's actual floating point representation.

src/modp_numtoa.c

@@ -23,6 +23,8 @@
  */
 static const double _pow10[] = {1, 10, 100, 1000, 10000, 100000, 1000000,
                                10000000, 100000000, 1000000000};
+static const double _pow10r[] = {1, .1, .01, .001, .0001, .00001, .000001,
+                                .0000001, .00000001, .000000001};
 
 static void strreverse(char* begin, char* end)
 {
@@ -287,6 +289,14 @@ void modp_dtoa2(double value, char* str, int prec)
         prec = 9;
     }
 
+    double smallest = _pow10r[prec];
+
+    if (value != 0.0 && value < smallest) {
+        sprintf(str, "%.*e", DBL_DECIMAL_DIG - 1, neg ? -value : value);
+        sn_strip_trailing_zeros(str);
+        return;
+    }
+
     int whole = (int) value;
     double tmp = (value - whole) * _pow10[prec];
     uint32_t frac = (uint32_t)(tmp);

src/modp_numtoa.h

@@ -91,6 +91,10 @@ void modp_dtoa(double value, char* buf, int precision);
  * will be switched exponential format and include as many precision digits
  * as needed to preserve information.
  *
+ * If a non-zero input value is less than 10^(-precision), the output format
+ * will be switched exponential format and include as many precision digits
+ * as needed to preserve information.
+ *
  * \param[in] value
  * \param[out] buf  The allocated output buffer.  Should be 32 chars or more.
  * \param[in] precision  Number of digits to the right of the decimal point.